1. Field of the Invention
The present invention relates to a control system for an automatic transmission and, in particular, to a control system for an automatic transmission which works as an engine brake effectively by holding a lower shift gear stage.
2. Description of Related Art
Japanese Laid-Open Publication Patent No. 2-38747 discloses a control system for an automatic transmission which can work as an engine brake effectively by holding lower shift gear stage thereof when a driver requires a reduced vehicle speed. Namely, in this conventional control system, a 3-4 upshift line is extended to 75 km/h where the throttle valve opening is near zero, for example, as shown in a shift line map in FIG. 2. Further, when the driver closes the throttle valve opening at the third shift gear stage in order to reduce a vehicle speed as shown by a symbol .beta. in FIG. 2, and then the driving condition goes across the 3-4 upshift line, the upshift operation to the fourth shift gear stage is delayed for a predetermined time. Thereafter, when the driving condition goes into the extended zone of the lower shift gear stage before the predetermined has passed, the upshift operation to the fourth shift gear stage is prohibited and the third shift gear stage is held so that the engine brake works effectively.
However, in the conventional control system for the automatic transmission, when the throttle valve opening is closed a little and the opening needs to be held as shown by a symbol .alpha. in FIG. 2, the upshift operation to the fourth shift gear stage is not carried out immediately after the driving condition goes across the 3-4 upshift line and, then, the upshift operation is delayed for the predetermined time. As a result, the driver feels uncomfortableness in the upshift operation of the automatic transmission.
On the other hand, in the conventional control system for the automatic transmission, when the throttle valve opening is fully closed, the engine brake works effectively by holding the lower shift gear stage. Thereafter, when the driver operates an accelerator pedal, the holding of the third shift gear stage is released. However, if the driving condition goes across the 3-4 upshift line from the extended zone of the third shift gear stage, the upshift operation to the fourth shift gear stage is carried out immediately after the driver operates the accelerator pedal. As a result, the driver feels uncomfortably under this situation.